In electrostatographic-type copiers and printers in common use, a charged imaging member such as a photoconductive insulating layer of a photoreceptor may be electrically charged and thereafter exposed to a light image of an original document or a laser exposure of a digitally stored document. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with toner. During development, the toner particles are attracted from carrier particles by the charge pattern of the image areas on the photoconductive insulating surface to form a powder image on the photoconductive insulating surface. This image may be subsequently transferred to a support surface such as a copy substrate to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle. The imaging processes described above are well known in the art.
Various types of charging devices have been used to charge or precharge charge retentive surfaces such as the photoconductive insulating layers of photoreceptors or such as copy substrates prior to transfer of toner images. These charging devices include corotrons, dicorotrons, pin corotron, scorotron, discorotron, and pin scorotron. See, generally, R. M. Schaffert, “Electrophotography,” The Focal Press, New York, 1965.
A scorotron device, included within the list above, it typically comprised of one or more corona wires or pin arrays with a conductive control grid or screen of parallel wires or apertures in a charge plate positioned between the corona producing element and the photoreceptor. A potential is applied to the control grid of the same polarity as the corona potential but with a much lower voltage, usually several hundred volts, which suppresses the electric field between the charge plate and the corona wires and markedly reduces the ion current flow to the photoreceptor.
The pin array variety of scorotron has proved to be a particularly inexpensive, durable, and effective device. Pins are often formed by forming “saw teeth” in a conductive metal sheet mounting these saw teeth edgewise facing the scorotron grid. In this arrangement, however, certain difficulties have been observed. One such difficulty is a sinusoidal wave pattern of charging thought to result from the increased charge potential located at the peaks of each pin when compared to each “valley” between pins. The scorotron grid is known to ameliorate the problem by diffusing the charge pattern through the grid pattern. Another method of ameliorating this problem is using at least two pin arrays arranged in parallel fashion such that the peaks of pins in the first array align with the valleys of the second array along the imaging path. Use of conventional scorotron grids with such dual pin arrays is known to produce charge uniformity across a process width of about plus or minus 25 volts for mid-range process speeds. In high quality printing, however, even relatively minor fluctuations in charge potential across the charged imaging surface, such as plus or minus 25 volts, cause undesirable printing irregularities.
A typical prior art scorotron device with dual pin arrays and a scorotron grid is shown in FIG. 1 (FIG. 1 is adapted from U.S. Pat. No. 4,725,732 which is hereby incorporated herein in its entirety.) In this perspective exploded view, scorotron charging device 100 is shown with two spaced apart, generally parallel pin arrays, 200 and 202, each supported on support projections 204. The distance between arrays 200 and 202 is chosen to be as large as possible consistent with the need for a compact device since smaller spacing between the arrays results in the need to increase power levels to drive the scorotron. Locator pin 208 is provided to correctly position pin array 202 while another locator pin (not shown) positions pin array 200 in a position offset by a spacing of ½pitch in order that each peak of pin array 200 laterally corresponds to a valley of pin array 202 and vice versa. Frame members 206, 238, 212, 230, and 214 contain the corona field emitted from pin arrays 200 and 202 while providing support and means for mounting the arrays. Scorotron grid member 247 attaches to appropriate frame members. Openings in grid member 247 enable the corona field to emerge from charging device 100 and to interact with the charge retentive elements of a charged imaging surface (not shown). Electrically insulated wire 222 conducts charging DC current to pin arrays 200 and 202 while insulated wire 220 conducts regulating current to grid member 247.
As shown in FIG. 2, charging device 100 is assembled into printing system 300. Typical uses within printing system 300 include charging of any charge retentive surface such as that of a photoreceptor 301 as shown in FIG. 2 or other imaging surface prior to image development as well as charging of a copy substrate 302 prior to toner transfer as well as detaching of the copy substrate 302 after toner transfer. Printing system 300 may be any number of electrostatographic imaging systems including, without limitation, electrophotographic monochrome or color systems and including without limitation printers, copiers, and various multifunctional systems.
One approach to improving charge uniformity using scorotron charging devices is set forth in U.S. Pat. No. 6,459,873, issued to Song et al., where a pair of scorotrons cooperatively charge the charged imaging surface. The first scorotron device initially charges the imaging surface to an intermediate overshoot voltage and the second scorotron device thereafter uniformly charges the imaging surface to the final voltage. Improved uniformity is created because the first scorotron device provides a generally high percent open control grid area (a range above 70% is claimed in Song) while the second scorotron device provides a generally lower percent open grid area (a range below 70% is claimed in Song). The higher percent of opening in the first scorotron grid correlates to a greater rate of charging, or slope, while the smaller percent of scorotron grid opening correlates to a lesser slope, or lesser rate of charging. The lesser slope of the second scorotron device enables more precise control of the charging process and, as a result, greater uniformity. Song is hereby incorporated herein by reference in its entirety.
The dual scorotron device taught in Song improves charge uniformity due to the differential in percentage of openings between the first and second grids. It would be desirable, however, to further improve charging uniformity.
One embodiment of the invention is a charging system for charging a charge retentive surface, comprising: at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension; and grid elements, interposed between said corona producing element and the charge retentive surface, wherein the grid elements are arranged generally parallel to each other along the width dimension and comprise differentiated grid feature patterns.
Another embodiment of the invention is an electrostatographic imaging system, comprising: a charge retentive surface having a width dimension; at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension; and grid elements, interposed between the corona producing element and the charge retentive surface, wherein the grid elements are arranged generally parallel to each other along the width dimension and comprise differentiated grid feature patterns.
Yet another embodiment of the invention is a method for charging a charge retentive surface having a width dimension, comprising: electrically charging at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension, sufficiently to emit a corona field; affecting the corona field by interposing, between the corona producing element and the charge retentive surface, grid elements that are arranged generally parallel to each other along the width dimension and that comprise differentiated grid feature patterns.